This invention relates generally to gas turbines and, more particularly, to an improvement in nozzleless centripetal turbine assemblies.
A typical centripetal flow gas turbine includes a housing or casing defining a rotor cavity, a rotor rotatably mounted on the housing within the cavity, a volute chamber around the housing, and a circular orifice between the volute chamber and the rotor cavity whereby motive fluid flowing through the volute chamber is directed against the rotor to effect rotation of the latter. Experimental and theoretical analysis of such turbine assemblies has shown that the absolute angle at which the motive fluid passes through the circular orifice and impinges on the rotor, known as the stator exit angle, has a direct and significant effect on the total efficiency of the turbine assembly. The optimum stator exit angle for any particular gas turbine depends upon the physical characteristics of the unit in question and can be determined through known theoretical and/or experimental techniques. Once a desired stator exit angle is established for a particular turbine design, maximum efficiency is obtained if that exit angle is maintained around the full 360.degree. of the circular orifice. One common method of achieving uniform stator exit angle is to form a nozzle around the rotor cavity by inserting stationary vanes in the circular orifice oriented at the desired angle, the nozzle thus functioning to direct the motive fluid at the proper angle. In more sophisticated installations the vanes are adjustable so that the exit angle can be tailored to the speed of the rotor to maximize efficiency over the working speed range of the turbine assembly. Such solutions, however, are not economically attractive for high volume production applications, such as automotive exhaust driven turbochargers, where simpler and less expensive nozzleless turbine assemblies have been traditionally used. Without a nozzle, however, efficiency deteriorates because there is less control of the stator exit angle. A nozzleless gas turbine assembly according to this invention includes provision for improving the stator exit angle and, therefore, represents an improvement over heretofore known nozzleless centripetal flow turbine assemblies.